Ultrasound technology may be commonly used for the purpose of detection and location of various objects [1], such as for fault detection in underground cables [2] and for medical imaging [3]. A common approach in imaging objects is to employ conventional piezoelectric transducers. However, piezoelectric transducers may have drawbacks that limit their applications, such as poor acoustic matching, dimensional limitation, temperature dependence, narrow bandwidth, and/or limited uniformity arising from fabrication difficulties. With the help of microfabrication techniques, microelectromechanical system (MEMS)-based ultrasound transducers have been introduced as an alternative to piezoelectric transducers [4, 5]. These devices, also known as capacitive micromachined ultrasonic transducers (CMUTs), may provide one or more advantages over traditional transducers. For example, they may offer wider bandwidth, better acoustic matching, higher sensitivity, highly miniaturized system, ability to produce large and uniform arrays with different number of cells, improved electrical safety, temperature independent properties, effective beam steering, and/or the potential for mass fabrication [5-9].
However, MEMS-based ultrasonic devices may still exhibit drawbacks, such as high driving voltage requirements, safety issues, and/or cavity and insulating layer breakdown due to the large electric field. Moreover, demands for high resolution imaging may result in a desire for generating even higher acoustic power and pressure, especially for imaging complex geometries such as multi-layer underground power cable. Higher sensitivity may also be desirable when operating in receiving mode, since the reflected wave can be weakened due to the wave passing through several layers of different material, as well as the distance of the object.